1. Field of the Invention
The present invention relates to a digital watermarking technology, and it particularly relates to an apparatus and method for embedding a digital watermark and an apparatus and method for extracting a digital watermark.
2. Description of the Related Art
The number of Internet users has rapidly increased in recent years and we are now entering the age of the broadband, or a new stage in the utilization of the Internet. Since communication bandwidth has greatly expanded in broadband communication, the distribution of items containing large bodies of data such as audio, still image, and video can be enjoyed with ease. When the distribution of such digital items becomes popular, a highly efficient method of protecting the copyright of their contents will be required.
In the present situation, the copyright is not protected well so that users can easily copy such contents distributed via the Internet. Therefore, technology for embedding information on the originator of the content and the user into the content as a digital watermark has been developed. By using this watermarking technology, it becomes possible to extract the digital watermark from the content distributed via the network, and thereby detect an illegal use and track the distribution route of an illegal copy.
In some conventional digital watermarking techniques, a digital watermark is embedded and thereafter meta-information for identifying the watermarking method of the digital watermark is embedded. For instance, see the references [1]-[3].
A double watermarking system in which different watermarking methods are utilized in a hybrid manner has been also proposed, for instance, in the reference [4].
A digital watermark is embedded in the content data in a way that the watermark is perceptually invisible to users in order to prevent malicious attacks. However, the content data will be subject to various operations during the distribution and utilization, for instance, signal processing such as compression, various types of filtering, modification by users, or attacks on the watermark information. In such processes, a part of the digital watermark may be possibly altered or removed. Therefore the digital watermark should be robust enough to withstand such operations.
In the references [1]-[3], some double watermarking methods are proposed by which two digital watermarks are embedded and the two watermarks are sequentially extracted, however, the two watermarks generally interfere with each other and therefore in some cases the watermarks cannot be extracted correctly. In the reference [4], a hybrid two-layered digital watermarking system is proposed to reduce the interference of the two watermarks. However, since the system limits the watermarking method available in the lower layer, it is not applicable to a general use.
Related art list:
[1] JP 2002-16891 A
[2] JP 2000-287067 A
[3] JP 2001-257865 A
[4] Oogami et al., “Hybrid Two-Layered Digital Watermark”, ITE Annual Convention 2002.